Electrochemical devices using liquid electrolytes have a safety problem because of the possibility of leakage and explosion. Electrochemical devices using polymer electrolytes were developed to solve this problem. For example, a lithium-polymer battery is superior in safety and also provides excellent charge-discharge efficiency. In addition, various designs are possible and the cell size can be reduced because the electrolyte can be obtained in a thin film.
Particularly, solid polymer electrolytes which comprise polyalkylene oxide (PAO)-based and gel-type polymer electrolyte containing organic liquid electrolytes were viewed as candidates of polymer electrolytes of lithium secondary batteries. Efforts have been made to improve conductivity of polymer electrolytes by adding low-molecular-weight polyalkylene oxides or organic solvents as a plasticizer. However, physical properties of polymer electrolytes deteriorate significantly or a stable gel may not be obtained if the content of a plasticizer increases.
A method to overcome the above-identified problem was disclosed, wherein a crosslinked polymer electrolyte was prepared by curing a composition containing polyalkylene glycol compound having a chemically crosslinkable group, an ion-conductive liquid and an electrolyte salt through UV radiation or electron beam radiation [U.S. Pat. No. 4,830,939, J. Electrochemm. Soc., 145, 1521 (1998)].
Recently, researches have been conducted using polysiloxane-based polymers, which have good flexibility and low glass transition temperature, and are thus expected to be capable of improving molecular chain movement, as the backbone of polyalkylene oxide-based, crosslinked polymer electrolytes [Macromolecules, 36 (2003), 9176, U.S. Pat. Nos. 4,673,718, 4,766,185, 5,227,043 and 5,440,011, Japanese Patent Laid-Open No. Hei 5-290616]. However, the polysiloxane-based polymer electrolytes are not suitable for use in lithium cells to be operated at room temperature, because they have poor mechanical properties and very low ion conductivity of 10−4 S/cm.
To solve the above problems, the present inventors developed a technique of partially enhancing mechanical properties by using a novel crosslinking agent obtained by introducing an acryl group, which is crosslinkable by heat or light, at both ends or side chain of a methylsiloxane polymer having polyalkylene oxide branches and acquired a patent for the technique [Korean Patent No. 419864, Korean Patent Application No. 2004-41502]. Although this polymer electrolyte composition was a fairly innovative one, it had a drawback that mechanical properties of the electrolyte cannot be controlled as required. This is due to the inability to control the exact control of degree of crosslinking because the content of the acryl group, which determines the degree of crosslinking, is dependent on the molecular weight of the polymer.
While endeavoring to solve the aforementioned problems, the inventors of the present invention discovered that a cyclic siloxane-based compound, when introduced with polyalkylene oxide acrylate groups, becomes easily crosslinkable and also its degree of crosslinking can be easily controlled. Further, if the above compound is used as a crosslinking agent of the polymer electrolyte it enables to control mechanical properties of the polymer electrolyte.
Therefore, in an embodiment of the present invention, there is provided a cyclic siloxane-based compound into which polyalkylene oxide acrylate groups are introduced.
In another embodiment of the present invention, there is provided a solid polymer electrolyte composition containing the cyclic siloxane-based compound as a crosslinking agent.
In still another embodiment of the present invention, there is provided a use of the solid polymer electrolyte composition as polymer electrolyte of electrolyte films and lithium-polymer secondary batteries.